(1) Field of the Invention
This invention relates to a method of removing a small amount of chlorine contained in converted potassium sulfate. More particularly, this invention relates to a method of removing a small amount of chlorine contained in converted potassium sulfate in which method portions, having such particle sizes as to remain on a screen of 60 mesh, of converted potassium sulfate obtained by a dry process are pulverized and calcined.
(2) Description of the Prior Art
Converted potassium sulfate produced by a dry process is theoretically obtained by mixing and heating 2 moles of potassium chloride with one mole of sulfuric acid as a whole to form 2 moles of hydrogen chloride as a by-product as shown in the following equations (1) and (2): EQU KCl+H.sub.2 SO.sub.4 .fwdarw.KHSO.sub.4 +HCl (1) EQU KCl+KHSO.sub.4 .fwdarw.K.sub.2 SO.sub.4 +HCl (2)
Potassium hydrogensulfate melts above 200.degree. C. and free sulfuric acid is also liquid. Therefore, the presence thereof in a large amount is liable to cause the reaction mixture to stick to an inner wall of a converter or to a stirrer in a reactor or a converter, resulting in making it difficult to attain an effective stirring and consequently a satisfactory reaction rate. In order to overcome the above difficulty, Japanese Patent Laid-open No. 17921/1981 (hereinafter referred to as a prior art method) discloses such a method to solve the problem of sticking as above (A) that more than an equivalent amount, that is, 2.0 to 2.2 moles of potassium chloride to one mole of sulfuric acid, is used, and (B) that the reaction in the first stage as shown in the equation (1) is carried out below 100.degree. C. and subsequently in the reaction of the second stage shown in the equation (2), potassium chloride and potassium hydrogensulfate are mixed for heating in the presence of potassium sulfate as the final product as well as in the presence of a minimum amount of unreacted sulfuric acid.
According to the prior art method, however, potassium sulfate as the product often contains residual chlorine (contained in the form of KCl) in an amount of from about 2.0 to about 4.5% by weight, because more than an equivalent amount to sulfuric acid (or potassium hydrogensulfate) of potassium chloride is used. Therefore, converted potassium sulfate made by the prior art method as above could not be used as potassic fertilizer for tobacco, where the chlorine content therein is required to be below 2% by weight, particularly below 1% by weight. Consequently, the present inventors repeated the reaction of the second state shown by the equation (2) in the prior art method (A) by adding for mixing concentrated sulfuric acid in such an amount that the combined amount thereof with the residual free sulfuric acid is equivalent to the amount of the residual chlorine in the converted potassium sulfate obtained by the prior art method as above, or (B) by adding for mixing concentrated sulfuric acid in such an amount as to be two fold the amount added in (A) as above. The results show that it is difficult to make a residual chlorine content in the retreated potassium sulfate less than 2.0% by weight in the former case (A) as above, and that free sulfuric acid is contained in the retreated product in a considerable amount of from 3 to 4% by weight, though the residual chlorine content can be reduced to 1.0% by weight or so in the latter case (B) as shown in Table 1.
TABLE 1 ______________________________________ Results of conversion reaction by addition of concentrated sulfuric acid: Reference Examples and Raw Sample No. Material Component (content) I II III ______________________________________ Raw Free sulfuric acid (%) 2.5 3.0 2.8 Material Residual chlorine content 3.5 4.0 3.8 (%) (A) Amount of concentrated 2.33 2.53 2.44 sulfuric acid added (%) Residual chlorine content 2.0 2.5 2.2 after reaction (%) (B) Amount of concentrated 4.66 5.06 4.88 sulfuric acid added (%) Residual chlorine content 0.9 1.1 1.0 after reaction (%) Residual sulfuric acid 3.55 4.03 3.80 content after reaction (%) ______________________________________
The reasons why the residual chlorine content after reaction in Reference Example (A) is not reduced as expected are considered due to (a) the fact that less content of residual or added sulfuric acid compared with that of the product as potassium sulfate makes difficult satisfactorily uniform mixing thereof with the product, and (b) the fact that a chemical structure in the form of pyrosulfuric acid of the residual free sulfuric acid makes impossible to react readily with potassium chloride containing therein chemically the residual chlorine.
In the case of Reference Example (B), the residual chlorine content is satisfactorily reduced, but a high content of free sulfuric acid prior to the retreatment as above requires additional steps for removal or neutralization thereof to be inefficient and uneconomical.
On the other hand, in place of the prior art method mentioned above, the use of sulfuric acid in an excess thereof, that is, in an amount of more than one mole to 2 moles of potassium chloride increases the content of unreacted sulfuric acid in the reaction shown by the equation (2) as above, and consequently increases sticking of the reaction mixture to the inner walls of the reactor as well as agglomeration of the reaction mixture with the result that the content of the unreacted potassium chloride is not reduced contrary to expectations.
In any case, an industrially efficient production of converted potassium sulfate by dry process, which contains the residual chlorine in an amount less than 2.0% by weight, preferably less than 1.0% by weight, has been found difficult without allowing the free sulfuric acid to be contained in a considerable amount of from 3 to 5% by weight, for example.
The present inventors made intensive studies to solve the technical problems as above and accomplished this invention by finding (a) that the residual chlorine content represented by % by weight (hereinafter % all means % by weight) in the potassium sulfate obtained from the converter for the second stage of the prior art method greatly differs according to particle size such as lump or granule of larger particle size or powder, (b) that the portion thereof in the form of lump or granule of larger particle sizes presents in an amount of from 20 to 25%, (c) that the portion in (b) is readily pulverizable to a certain powder, (d) that the portion thus pulverized may either be calcined at 300.degree. to 500.degree. C. for a short period of time to readily reduce the residual chlorine content to less than 3.5%, or may be mixed with a small amount of water or dilute sulfuric acid prior to calcination as above to readily reduce the residual chlorine content to less than 2.5%, and (e) that the portion in the form of powder of potassium sulfate product in (a) as above has such a residual chlorine content less than 1.0% and a very low content of free sulfuric acid as to be obtained as a converted potassium sulfate product of low chlorine content.